Well, they're not really wrong. HP are abstract. Full stop. Moldvay Basic defines HP as a pure abstraction where the HP you have is just a number until your character is dead. FWIW, I think that's the most honest description of HP in D&D.
Because the combat mechanics of D&D can never actually tell you anything about what is happening in the fiction of the game... hang on. Let me rephrase that. The combat mechanics of D&D can never actually tell you how anything happens in the fiction of the game (it can tell you what happened - you died) so adding in something like Armor DR presumes a correlation between the mechanics and the narrative that has never existed.
Orc hits character for 5 HP. What happened? You cannot actually tell me anything. Nothing in those mechanics tell me how that character lost 5 HP. Sure, you can craft a dozen different narratives that describe the HP loss, but, the mechanics don't actually prove or disprove any of those narratives.
Thus, adding in something like DR for armor ignores the primary basis of HP - that HP are an abstraction and, as such, defy reification.